1. Field of the Invention
The present invention relates generally to microelectromechanical (MEMS) devices and, in particular, to arrayed magnetically actuated MEMS devices such as arrayed mirrors used in optical switches and other devices like scanners and projectors.
2. Description of Related Art
FIG. 1 schematically illustrates an example of an optical cross-connect 12 of an optical switch. The cross-connect 12 includes an array of collimators or other beam-forming devices, represented by grid 14, and forms incoming optical communications signals into beams that impinge on an array of selectively moveable reflectors or mirrors represented by grid 16. Each beam from grid 14 has its own corresponding moveable mirror on grid 16.
The moveable mirrors of grid 16 are controllably positioned so as to individually direct the respective beams from grid 14 to respective moveable mirrors of a second array of moveable mirrors, represented by grid 18. The moveable mirrors of grid 18 are positioned so as to individually direct the beams received from grid 16 to respective beam receivers of an array of beam receivers represented by grid 20. The beam receivers may take various forms, such as transducers, lenses or optical elements for coupling the respective beams into respective optical fibers, waveguides, or the like. As with grids 14 and 16, each moveable mirror of grid 18 is associated with a particular beam receiver of grid 20, so that each receiver receives beams on a single axis. A representative signal path from grid 14 to grid 20 is indicated by arrow 22.
Attempts have been made previously to fabricate arrays of mirrors such as those represented by grids 16 and 18 using MEMS technology, in which silicon processing and related techniques common to the semiconductor industry are used to form microelectromechanical devices. For switches such as those shown in FIG. 1, it is desirable to have an array of moveable mirrors that are both densely packed and easily controlled.
As is known in the art, movable mirrors can be actuated or controlled in a variety of ways including through electromagnet actuation, electrostatic actuation, piezoelectric actuation, stepper motors, thermal bimorph and comb-drive actuation.
FIGS. 2A and 2B illustrate an electromagnetically actuated single-mirror device 30 in accordance with the prior art. The device 30 includes a mirror structure 32 movably supported on a gimbal structure, which comprises inner and outer gimbal frames 34, 35. The mirror structure 32 includes a mirror having a reflective surface 33, which is on the same side of the mirror structure as an actuation coil 36.
Two actuation coils are provided: the inner coil 36 on the mirror structure 32, and an outer coil 38 on the inner gimbal frame 34. An external magnetic field B oriented at 45 degrees to the X and Y axes provides torque when either the inner or outer coils are actuated with current, thereby causing the mirror structure 32 to rotate about respective torsional hinges or flexures 40, 41 as desired.
The actuation coil 36 on the mirror structure 32 requires space and reduces the area available for the reflective surface area of the mirror. (Consequently, the mirror area fill factor of the device, which is the ratio of the area of the exposed reflective surface 33 of the mirror relative to the total area of the device 30, is relatively small.) Having small mirrors is undesirable because they cannot intercept as much of an optical beam directed thereto, causing higher insertion losses. Alternatively, larger mirrors can be used, but with reduced packing density, which is the number of arrayed mirror devices in a given area.
The magnetic field applied to mirror devices of the type shown in FIG. 2 can be provided by magnets positioned in the plane of the mirror. As shown, e.g., in FIG. 3, a mirror device 42 includes magnets 44 in the plane of the mirror structure 32. A strong magnetic field is needed at the plane of the mirror and gimbal to reduce the current needed to deflect the mirror, and thereby reduce power consumption and heating of the mirror. The magnets 44 are accordingly relatively large. A frame 46 of soft magnetic material is often provided to intensify the field. The relatively large magnets and frame make it difficult to have multiple mirrors of this type positioned close to each other in an array thereby reducing the packing density of such arrays.
A need accordingly exists for movable mirror devices having a high mirror area fill factor, and which can be densely packed. A need also exists for a method of manufacturing such devices. A need further exists for mirror devices that can be easily and accurately controlled.